Let’s say you drive the a 300 mile distance with two cars. First one does 45 mpg and the other one does 40. Seems like a small difference, and it is. Once you reach the destination, and compare how much money you spent on gas, the difference is probably something like 3.6 dollars.

Next, you do the same trip with a different set of cars: 20 and 25 mpg. That was a small difference, right? Not any more. At the end of the trip you compare the receipts and find that it’s a 13.2 dollar difference now.

As gas is the material being consumed and miles is what you get in return, having them in the mi/gal configuration implies you should care a lot more about the miles. However, the distance you need to drive usually isn’t something you think about that much. Instead, people care much more about how much you have to spend on gas. The volume of gas connects with how much you end up spending, and that’s why you should put it on top, not below.

If gas was nearly free, an all you cared about is distance, mpg would be ideal.